The combustion of coal and other fossil fuels, as is known, produces flue gases that contain sulfur dioxide. In addition, nitrogen oxides are also produced which are carried in the flue gases. While efforts for reducing pollution caused by such flue gases were initially concentrated to removing the sulfur dioxides, recent regulations have also provided that the nitrogen oxide content of flue gases, prior to discharge to the atmosphere, must also be lowered.
Aqueous scrubbing slurries are known which use calcium components such as lime for removing sulfur dioxide. Especially useful such slurries formed from calcium hydroxide and magnesium hydroxide have been found to be very effective in the removal of sulfur dioxides from flue gases in wet scrubbers. Such wet scrubbing processes are illustrated by the teachings of U.S. Pat. Nos. 3,919,393, 3,919,394 and 3,914,378, all assigned to the assignee of the present invention.
The removal of nitrogen oxides from flue gases, however, continues to provide problems in wet scrubbing systems. While the use of ferrous ethylenediaminetetracetic-acid (ferrous-EDTA) has been found to be effective in acting as a promoter to remove nitrogen oxides from flue gases, a problem exists in that ferrous-EDTA oxidizes to ferric-EDTA in systems where oxygen is present and the ferric-EDTA is not reactive for nitrogen oxide removal. Examples of such a process, where sulfur dioxide and nitrogen oxides are removed from flue gases are described in U.S. Pat. No. 4,612,175 and U.S. Pat. No. 4,670,234, the contents of both said patents incorporated by reference herein.
It would be beneficial to use ferrous-EDTA in a sulfur dioxide scrubbing system to simultaneously remove nitrogen dioxides from a flue gas. However, such retrofit technology has not been successfully applied to commercial-scale scrubbing systems, primarily because the ferrous-EDTA, being readily oxidized to ferric-EDTA, is rendered ineffective in such oxygen-containing systems.
Attempts to use ferrous-EDTA as a nitrogen oxide remover in wet flue gas desulfurization systems also results in a buildup of nitrogen oxide reaction products in the liquid phase in the flue gas desulfurization systems. In addition, discharge of these reaction products in the sludge removed from the scrubbing system causes problems.
It is an object of the present invention to provide a process for simultaneous removal of sulfur dioxide and nitrogen oxides in a wet scrubbing system, where ferrous-EDTA is used as a promoter, which process minimizes loss of the expensive ferrous-EDTA, minimizes the buildup of nitrogen oxide reaction products in the liquid phase of the scrubbing system, and removes reaction products and most of the ferrous or ferric-EDTA from the scrubbing slurry before the slurry is concentrated and discharged as a solid waste for disposal.